(1) Field of the Invention
The present invention relates to a dehydrating and water-retaining sheet. More particularly, it relates to a multipurpose dehydrating and water-retaining sheet which can be used for various applications and can be used in a disposable way.
(2) Description of the Related Art
Processes whereby foods are dehydrated and dried to make them keepable have been known for many years. For example, keepable foods have been prepared by various methods, such as heating, vacuum treatment, solvent treatment, freeze-drying, preserving in salt, and preserving in sugar. Recently, keepable foods having an intermediate water content have been developed by the discovery of a contact dehydration method using a dehydrating sheet comprising a combination of a water-permeable sheet and a water-absorbing agent, and these kept foods have become popular.
This contact dehydration method is advantageous in that dried fish having a low salt content can be obtained because no salt is used, the propagation of bacteria can be prevented during the production process of the dried fish because the dehydration proceeds at a significant speed, no deterioration in the protein occurs because there is no application of heat, self-digestion due to enzymes is prevented, and the production of the dried foods does not require a large-scale apparatus.
However, the sheets for contact dehydration prepared hitherto have various defects and are not satisfactory for practical purposes.
For example, Japanese Unexamined Patent Publication (Kokai) No. 56-75054 discloses a dehydrating sheet comprising a combination of a semipermeable membrane such as a collodion membrane and a water-absorbing agent consisting of an organic compound. Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 56-124404 discloses a dehydrating sheet comprising a combination of the above-mentioned semipermeable membrane and a high tension organic substance such as sucrose.
The semipermeable membrane is however disadvantageous in that the bonding strength is low and the tensile strength is remarkably variable depending upon the water content.
Japanese Unexamined Patent Publication (Kokai) Nos. 55-21203, 57-122914, 57-167734 and 58-107133 disclose a combination of a water-permeable membrane, such as a heat-sealable unwoven fabric or a perforated film, and an absorbing agent. Also known is a dehydrating sheet comprising the above-mentioned combination and a substrate consisting of a water-absorbing agent (Japanese Unexamined Patent Publication (Kokai) No. 57-167734) or a dehydrating sheet with quilting applied to the entire surface thereof (Japanese Unexamined Patent Publication (Kokai) No. 55-21203). These dehydrating sheets are disadvantageous in that, because the water absorption occurs only by a capillarity between the water-absorbing agent particles through the water-permeable membrane, the rate of water absorption is slow, and further, because the water absorption occurs only at portions of the dehydrating sheet in contact with the object to be dehydrated, the water absorbing agent undergoes an uneven and partial expansion which hinders the attainment of the desired object.
In view of these facts, a combination of a high tension liquid such as highly condensed sucrose and thick melt syrup and a polymeric water-absorbing agent is known from Japanese Examined Patent Publication (Kokoku) No. 58-58124. In accordance with this dehydrating sheet, which is prepared by applying the high tension liquid on the water permeable membrane, the defect of the partial contact of the above-mentioned prior art is improved and the contact area becomes large, with the result that the dehydration of goods can be efficiently carried out. However, this dehydrating sheet is disadvantageous in that, because of the high cost involved, it should be able to be reused. However, this means that the used sheet should be dried before reuse, and thus sanitary problems arise, especially when it is used for dehydrating foods, and therefore, various limitations are imposed when it is to be reused.
A combination of a water-absorbing polymer substance and hydrophilic adhesive substance in which the water-absorbing polymer substance is dispersed in the hydrophilic adhesive substance is known from U.S. Pat. No. 4,383,376. This dehydrating sheet, however, absorbs liquid water but does not absorb gaseous water.
With this structure of the above-mentioned dehydrating means, it is considered that a stage in which water is absorbed in the water-absorbing agent due to capillarity at a stage in which water is absorbed in the high tension liquid due to a difference in osmotic pressure determines the rate of dehydration.